The present invention relates to an improved mercury reed switch operating in the so-called Form C mode, that is a switch having a break-before-make combination or single-pole, double-throw combination.
A mercury reed switch comprises a reed armature having its base immersed in a pool of mercury and having its upper end movable between two stationary contacts. The capillary action allows the mercury to flow over the reed surface, thereby wetting the contacting surfaces of the armature and the contacts, giving actually a mercury-to-mercury contact which has a very low contact resistance.
The prior art for a break-before-make mercury switch is illustrated in FIG. 1. Within a sealed capsule, the armature is formed of a cantilever beam 1 fixed at its lower end which is immersed in the mercury pool 2. The armature is adjusted in a neutral position between the contacts 3 and 4 so that the armature acts as a spring without initial pressure. The normally closed condition is obtained by using a permanent magnet externally fixed to the capsule. By adjusting the position of the magnet, the arrangement as described can operate in a normal monostable switching mode (with free return of the armature to the normally closed condition after deenergization. or a latching mode (bistable action with return of the armature to the normally closed condition only in response to an inverse energization). In FIG. 1, the magnet P is positioned for creating a bistable mode of operation. In either case, the function of the cantilever beam is to provide enough mechanical retractile force around a fixed oscillation point. A drawback of such an arrangement is that the switch must be used in a vertical position and this restricts its use to those cases where this condition is met. Furthermore such a capsule can hardly be miniaturized and it is relatively expensive.